1. Field of the Invention
The present invention relates to a duplexer in which a transmission filter chip and a reception filter chip are mounted on a substrate and, more specifically, to an acoustic wave duplexer in which each filter chip is an acoustic wave filter chip, such as a surface acoustic wave filter chip or a boundary acoustic wave filter chip.
2. Description of the Related Art
Traditionally, for a mobile communication system, such as a cellular phone, in order to reduce the number of components, a composite component having multiple functions is highly desired. As one such example of a composite component, a duplexer is known to include a reception filter chip used in a reception circuit and a transmission filter chip used in a transmission circuit.
For example, Japanese Unexamined Patent Application Publication No. 2003-249842 discloses, as one example of duplexers of this type, a surface acoustic wave duplexer illustrated in the diagrammatic circuit diagram in FIG. 17, described in more detail below.
In a surface acoustic wave duplexer 1001, a transmission acoustic wave filter chip 1003 and a reception acoustic wave filter chip 1004 are surface mounted on a substrate 1002. Each of the transmission acoustic wave filter chip 1003 and the reception acoustic wave filter chip 1004 has a first end connected to a common terminal 1005. The reception acoustic wave filter chip 1004 is connected to the common terminal 1005 through a phase shift circuit 1006. The common terminal 1005 is a terminal connected to an antenna or the like.
The transmission acoustic wave filter chip 1003 includes a plurality of series arm resonators S1 to S3 each made of a 1-port surface acoustic wave resonator and parallel arm resonators P1 and P2 each made of a 1-port surface acoustic wave resonator. The transmission acoustic wave filter chip 1003 has an end that is opposite to the first end connected to the common terminal 1005 and that is connected to a transmission terminal 1007. An electric signal to be transmitted is input from the transmission terminal 1007 and supplied to the common terminal 1005 through the transmission acoustic wave filter chip 1003.
Meanwhile, the reception acoustic wave filter chip 1004 has a structure in which first and second longitudinally coupled resonator surface acoustic wave filters 1008 and 1009 are connected in parallel. Each of the longitudinally coupled resonator surface acoustic wave filters 1008 and 1009 includes a first IDT arranged in its center. First ends of the first IDTs are commonly connected to the common terminal 1005 through the phase shift circuit. A second end of each of the first IDTs is connected to a ground potential.
First ends of second and third IDTs arranged at both sides of the first IDT of the longitudinally coupled resonator surface acoustic wave filter 1008 in a surface wave propagation direction are commonly connected to a first reception terminal 1010.
First ends of second and third IDTs of the longitudinally coupled resonator surface acoustic wave filter 1009 are also commonly connected to the first reception terminal 1010. The second and third IDTs of each of the longitudinally coupled resonator surface acoustic wave filters 1008 and 1009 are commonly connected to a second reception terminal 1011 serving as a second balanced terminal.
Accordingly, the above-described reception acoustic wave filter chip 1004 is a surface acoustic wave filter including the first and second reception terminals 1010 and 1011 and having the balanced-to-unbalanced conversion function.
For the surface acoustic wave duplexer 1001 described in Japanese Unexamined Patent Application Publication No. 2003-249832, the transmission acoustic wave filter chip 1003 forming the transmission filter and the reception acoustic wave filter chip 1004 forming the reception filter are surface mounted on the substrate 1002 to aim at providing a multifunction device. Accordingly, the number of components can be reduced. In addition, the reception acoustic wave filter chip 1004 has the balanced-to-unbalanced conversion function, as described above, so a balun can be omitted.
However, there is a problem in which isolation between the transmission acoustic wave filter chip 1003 and the reception acoustic wave filter chip 1004 is not sufficient. That is, the transmission acoustic wave filter chip 1003 has a ladder circuit configuration, and during transmission, a transmission current flows and a magnetic field occurs. For the balanced reception acoustic wave filter chip 1004, a reception current is conveyed from the first reception terminal 1010 being the first balanced terminal toward the second reception terminal 1011 serving as the second balanced terminal. However, in a transmission frequency band, an induced current caused by the above-described magnetic field flows while being superimposed on the reception current. Because of this, a problem is present in which the isolation characteristic in the reception acoustic wave filter chip 1004 deteriorates in the pass band of the transmission band-pass filter.